This invention relates to lithium ion batteries.
Rechargeable batteries based upon lithium ion cells are attractive because they have inherently high capacities, high energies, and are operable over a useful temperature range. Such batteries feature a cathode, an anode, and a liquid or solid organic electrolyte. One problem with such batteries, however, is a tendency for the battery to self-heat at elevated temperatures. Self-heating results when exothermic reactions within the battery components are activated. Sustained self-heating occurs when the rate of heat generation within the battery exceeds the rate of heat dissipation from the battery surface to the surrounding area. Sustained self-heating can lead to thermal run-away, resulting in venting, flaring, and, in some cases, explosion.
The rate of heat generation increases as the capacity and energy of the battery increase. Accordingly, the problem of sustained self-heating becomes increasingly important as the battery industry seeks to maximize the capacity and energy of lithium ion batteries.
The sustained self-heating problem also limits the ultimate size of the battery. This is because surface area grows as the square of the battery dimensions, whereas volume grows as the cube of the battery dimensions. Accordingly, at a certain size, the rate of heat generation within the battery exceeds the rate of heat dissipation from the battery surface, leading to sustained self-heating at elevated temperatures.
In general, the invention features a lithium ion battery that includes: (a) a cathode; (b) an anode in the form of a thin film; and (c) an electrolyte separating the anode and the cathode. Both solid and liquid electrolytes can be used. The battery has a capacity of at least 600 milliamp-hours, a specific energy of at least 100 watt-hours/kg, and a volumetric energy of at least 250 watt-hours/liter. When lithiated, the anode does not exhibit sustained self-heating at temperatures up to about 100xc2x0 C., preferably at temperatures up to about 150xc2x0 C., and more preferably at temperatures up to about 170xc2x0 C., as determined using Accelerated Rate Calorimetry described infra. Even more preferred are anodes that do not exhibit sustained self-heating at temperatures up to about 200xc2x0 C. Preferably, the anode includes an electrochemically active elemental metal.
A xe2x80x9cthin film anodexe2x80x9d refers to an anode in the form of a continuous film that is free of binders such as polymeric binders. Accordingly, such anodes are distinguishable from composite anodes prepared by admixing an electrochemically active powder with a conductive diluent such as graphite or carbon black, and a binder.
An xe2x80x9celectrochemically active elemental metalxe2x80x9d is an elemental metal that reacts with lithium under conditions typically encountered during charging and discharging in a lithium battery. An xe2x80x9celectrochemically inactive elemental metalxe2x80x9d is an elemental metal that does not react with lithium under those conditions.
xe2x80x9cElemental metalxe2x80x9d refers to both metals and to metalloids such as silicon and germanium.
A xe2x80x9cliquid electrolytexe2x80x9d includes both liquids and liquid-swollen gels.
Examples of preferred electrochemically active elemental metals include aluminum, silicon, tin, antimony, lead, germanium, magnesium, zinc, cadmium, bismuth, and indium. The anode may further include one or more electrochemically inactive elemental metals. Examples include molybdenum, niobium, tungsten, tantalum, iron, and copper. One useful anode features a combination of tin and molybdenum. Other useful anodes include silicon, alone or in combination with aluminum or tin.
The anode has a specific capacity of at least 100 milliamp-hours/g (preferably at least 300 milliamp-hours/g) and a volumetric capacity of at least 600 milliamp-hourslcm3. Several different anode compositions may be used.
According to one embodiment, the anode consists essentially of a plurality of electrochemically active elemental metals, and has a microstructure that includes these elemental metals in the form of a mixture that is essentially free of domains measuring greater than about 1000 angstroms. A xe2x80x9cdomainxe2x80x9d is a region that consists essentially of a single electrochemically active elemental metal. The domain may be crystalline (i.e., it gives rise to a discernible electron or x-ray diffraction pattern characteristic of a crystalline material) or non-crystalline. The size of the domain refers to the longest dimension of the domain. Examples of such anodes are described in Turner, U.S. Ser. No. 09/113,385 entitled xe2x80x9cElectrode Material and Compositions Including Same,xe2x80x9d filed Jul. 10, 1998 and assigned to the same assignee as the present application, which is hereby incorporated by reference in its entirety.
A second useful anode composition is one in which the anode includes (a) an electrochemically active elemental metal and (b) an electrochemically inactive elemental metal. The microstructure of the anode is characterized by the presence of crystalline regions after a battery incorporating the anode has been cycled through one full charge-discharge cycle.
These crystalline regions, which are characterized by a discernible x-ray diffraction pattern characteristic of a crystalline material, preferably have at least one dimension that is no greater than about 500 angstroms after the battery has been cycled through one full charge-discharge cycle, and do not substantially increase after a total of at least 10 cycles. Moreover, these crystalline regions are preferably separated by regions that include the electrochemically active elemental metal and the electrochemically inactive elemental metal in which the relative proportions of the electrochemically active elemental metal and the electrochemically inactive elemental metal vary throughout the thickness direction of the composition (i.e., the direction perpendicular to the substrate on which the thin film is deposited). These latter regions do not exhibit an electron diffraction pattern characteristic of a crystalline material. They may be present prior to cycling, after cycling, or both before and after cycling. Examples of such anodes are described in Turner et al., U.S. Ser. No. 09/048,407 entitled xe2x80x9cElectrode Compositions,xe2x80x9d filed Mar. 26, 1998 and assigned to the same assignee as the present application, which is hereby incorporated by reference in its entirety.
The battery also preferably exhibits good high temperature properties. Specifically, after being subjected to one full charge-discharge cycle at 80xc2x0 C., the capacity fade is no greater than 2% per cycle, preferably no greater than 1% per cycle, and, even more preferably, no more than 0.5% per cycle.
The invention provides lithium ion batteries that exhibit high capacity and energy, yet exhibit improved resistance to sustained self-heating relative to batteries featuring graphite- or carbon-containing composite electrodes. Conventional battery designs can be used, thereby eliminating the need for elaborate mechanical measures. Moreover, even batteries having relatively large dimensions can be manufactured. The batteries also retain their superior performance (as measured by the extent of capacity fade) at high temperatures.
These batteries could be used as a power supply for a number of devices that operate using energy from a power supply. Examples includes vehicles such a automobiles, trucks, and bicycles. The batteries also could find numerous applications in aircraft. For example, they could be used as an engine starter for the aircraft. They could also be used to supply power to on-board computers and telephones. Other potential applications include power supplies for satellites, telecommunications devices such as cellular telephones, and portable computers.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.